Local currents at Higher Level: An AHL statement: how an action potential moves along an axon.
Practise this as you read
- Explain propagation by local currents.
- Explain how an impulse is passed along a neuron.
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The big idea: An action potential moves along an axon because of local currents. Where the membrane is depolarized, sodium ions diffuse along inside the axon to the next region, and outside they diffuse back the other way. This makes the next region less negative until it reaches the threshold: its sodium channels open, and the action potential has moved on.
The points to remember
- At a depolarized region the inside is positive.
- Inside, Na⁺ diffuses along to the next region.
- Outside, Na⁺ diffuses back towards the depolarized region.
- The next region becomes less negative and reaches the threshold.
- Its sodium channels open: the impulse moves on, renewed each time.
Remember it as: Spill over, reach the threshold, fire, repeat.
Real example: why the impulse does not fade. Because each region fires its own full action potential, an impulse reaches the end of a metre-long axon to the foot as large as it started.
Trap: The local current only brings the next region to the threshold; the full action potential there comes from that region's own sodium channels opening.
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How it is asked: Expect the long Explain question on how an impulse is generated and passed along a neuron, or an MCQ on what brings the next region to the threshold.
IB-style questionExplain[3 marks]
Explain how local currents propagate an action potential along an axon.
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